We have, in previous studies, made a discovery that generalized transducing Salmonella phages recombine with various unrelated bacteriophages to evolve into new hybrid phage species. Recently, we isolated an unusual Salmonella typhimurium-E. coli hybrid with sensitivity to coliphage lambda and Salmonella phage P22. This bacterial strain and its derivatives have facilitated construction of new hybrid phage species between the evolutionary diverse phage lambda, which has a unique genome, and phage P22 which has a circularly permuted genome. Two broad classes of hybrid phages were characterized. The lambda-P22 hybrid class, which has the protein coat of lambda, contains at least the c region of P22. The P22-lambda hybrid class has the protein coat of P22 and has inherited at least the c marker of lambda. These hybrid phages provide an excellent model for studying a mechanism of genetic evolution, control of gene expression within gene clusters derived from diverse phages, phage morphogenesis, chromosome structure and nature of transduction. To understand the above mechanisms, we propose to study the following: (1) Physical Lengths of the Homologous regions between P22, P22-lambda, lambda-P22 and lambda. (2) Heteroduplex of Various Phage DNA Molecules. (3) Gene Control in lambda-P22 and P22-lambda Hybrids. (4) Isolation Repressor Molecules of P22 and lambda, Their In Vitro Binding to P22-lambda and lambda-P22 DNAs, and Effects on In Vitro Transcription. (5) An Unusual Hybrid between lambda and P22, Carrying both the lambda c Regions and the P22c Regions. (6) P22 Tail Gene Can Appear in lambda-P22 and P22l. (7) Function of the Im Gene of P22. (8) Location of Prophage Attachment Sites of P22-lambda and lambda-P22. (9) Origin and Orientation of DNA Replication of P22-lambda and lambda-P22. (10) Variation of Genome Lengths in Hybrids. (11) Genome Structure of lambda-P22 and P22-lambda Hybrids: Permuted or Unique chromosome. (12) Nature of Transduction by lambda-P22 or P22-lambda: Specialized vs. Generalized. (13) Genomic Masking vs. Phenotypic Mixing.